add dev-requirements.txt file to use with virtualenv

Description

As a developer new to Twisted, It was a bit unclear to me how I should proceed running Twisted's test suite during development.

It would be beneficial to new developers to be able to run a single command like pip install -r dev-requirements.txt to install all of the projects needed dependencies in a single step. This seems to be the norm amongst other Python projects.

To simplify the development of Twisted using virtual environments I propose that a dev-requirements.txt file be added to the repository. This file would list all requirements needed to be able to run twisted and it's test suite.

This was spawned from a discussion on irc.freenode.net/#twisted-dev at 13:23 CST on 12 March 2014.